Transformers generally form an integral part of a power transmission system, providing the possibility of converting one voltage level to another. Transformers in power transmission systems often represent huge investments, and are typically not manufactured until ordered. In case a transformer in a power transmission system needs to be replaced, it is often a lengthy process to get a replacement. In order to avoid situations where a transformer needs to be replaced in a rescue action, it is desirable to detect an arising problem at an early stage, so that required maintenance or planned replacement can be performed.
Transformer on-line monitoring systems have been designed to detect and indicate, while the transformer is in operation, deviations in properties of the transformer which could indicate a deterioration of the transformer status. Examples of properties that can be monitored are temperature, combustible dissolved gas, and bushing capacitance.
The transformer impedance is a further property which has been used in transformer diagnostics. Conventionally, transformer impedance has been determined off-line, i.e. from measurements performed when the transformer was disconnected from the power transmission system.
In US2010/0188240, it is proposed to monitor the transformer impedance while the transformer is in operation. The possibility of on-line impedance monitoring would be of great benefit, since the impedance of a transformer carries useful information on the transformer status, while disconnecting a transformer from the power transmission system in order to determine the impedance is costly. In US2010/0188240, it is proposed to obtain information on the transformer impedance by measuring the current A and the voltage E at the transformer terminals, and to generate (in a three phase system) a 9×9 impedance matrix from such measurements using the relations EiH−EjL=Zijaij and AiH=ai1+ai2+ai3, where aij represent transformer terminal currents; indices H and L indicate “high voltage side” and “low voltage side”, respectively; index i=1, 2, 3 indicates phases of the high voltage terminals; and index j=1, 2, 3 indicate phases of the low voltage terminals. As discussed in US2010/0188240, solving the equation system obtained by the proposed analysis introduces considerable computational complexity, and a simplification of the model is therefore proposed. However, even if such simplification is not employed, the accuracy of the results obtained by the proposed method will often not be sufficient for transformer diagnostic purposes.